An imaging apparatus with an image sensor using microwaves, millimeter waves, and terahertz waves is able to perform non-contact detection of a dangerous object or the like without causing harmful radiation exposure, differing from a visible optical sensor or an infrared sensor. For this reason, the imaging apparatus has been put into practical use in security checks at airports, event sites, and so on. The imaging apparatus using electric waves can detect a dangerous object, an unidentified object hidden under clothing or the like or an unidentified object behind a wall. In recent years, therefore, the imaging apparatus has been also used for non-contact size measurement in boutiques or the like.
Such an imaging apparatus has been called as a millimeter wave holographic system. The millimeter wave holographic system includes a plurality of communication devices arranged in a one-dimensional array (linear). The communication device includes, for example, transmitter, receiver or transmitter/receiver. Each communication device irradiates a millimeter wave to a target while performing a frequency sweep and then determines the intensity and phase of a reflection wave reflecting from the target for every sweep frequency. In the case of the one-dimensional array of transmission and receiving apparatuses, the measurement is performed by scanning the array in a vertical or horizontal direction and changing the spatial positions of the respective transmission apparatuses.
The reflectance f of the target at (x, y, z) can be represented by the following equation (1):
                              f          ⁡                      (                          x              ,              y              ,              z                        )                          =                              FT                          3              ⁢                                                          ⁢              D                                      -              1                                ⁢                      {                                          FT                                  2                  ⁢                                                                          ⁢                  D                                            ⁢                              {                                  s                  ⁡                                      (                                          x                      ,                      y                      ,                      w                                        )                                                  }                            ⁢                              ⅇ                                                      -                    j                                    ⁢                                                                                    4                        ⁢                                                                                                  ⁢                                                  k                          2                                                                    -                                              k                        x                        2                                            -                                              k                        y                        2                                                                              ⁢                                      z                    1                                                                        }                                              (        1        )            
Here, FT2D(x) represents a two-dimensional Fourier transform function on the scanning plane. FT3D−1(x) represents a three-dimensional inverse Fourier transform function. In addition, s(x, y, ω) represents a received power at a sweep (angle) frequency ω at a scanning position (x, y) and k represents a space wave number vector: 2K2=Kx2+Ky2+Kz2. In this holographic system, the relationship between a sweep frequency step Δf and a distance Rmax from the holographic system to the target can be represented by Δf<c/Rmax. Furthermore, c represents an electric wave propagation rate of a space medium. Therefore, the smaller the sweep frequency step Δf is, the more the ability to detect a substance in the distance can be increased. On the other hand, the detection power of this system is represented by the target in-plane direction δx≈λcF#/2 and the distance direction δx≈c/2B. Here, F# is a ratio of the distance R between the target and the communication device to the scanning length of the communication device. λc represents the wavelength of an electric wave and B represents the frequency bandwidth. In other words, the longer the scanning length is and the wider the frequency bandwidth is, the more the detection resolution increases.
Preferably, the communication device of the imaging apparatus may include two different high-frequency signals, a high-frequency signal (RF signal) to be used as a transmission signal and a local frequency signal (LO signal) to be used for down conversion of the received signal, which is the reflected signal of the RF signal. Thus, the traditional communication device has been designed to include two oscillators, a RF oscillator that generates a RF signal and a LO oscillator that generates a LO signal. Therefore, there is a disadvantage in that decreases in positional accuracy and detecting accuracy occur due to the phase noise of the signal source. The phase noise of the oscillator tends to be deteriorated in proportion to increase in frequency. Particularly, when the frequency being used is a millimeter wave or a sub-terahertz wave of higher than 90 GHz, such a disadvantage becomes remarkable. Accordingly, it is preferable to reduce the noise of the signal source because of the above reasons.
Here, the examples of the related art include those disclosed in Japanese Laid-open Patent Publication No. 11-311669, Japanese Laid-open Patent Publication No. 2006-203718, Japanese National Publication of International Patent Publication No. 2001-501304, Japanese National Publication of International Patent Publication No. 2009-526988, U.S. Pat. No. 5,455,590, U.S. Pat. No. 5,557,283, and D. Sheen, D. McMakin and T. E. Hall, “Three-Dimensional Millimeter-Wave Imaging for Concealed Weapon Detection” IEEE Trans. MTT, vol. 49, no. 9, pp. 1581-1592, 2001.